John Gill Commentary Acts 16:38

"And the sergeants reported these words unto the magistrates: and they feared when they heard that they were Romans;" — Acts 16:38 (ASV)

And the sergeants told these words unto the magistrates ,
&c.] They returned to them, and acquainted them with what the prisoners said:

and they feared when they heard that they were Romans ;
they were not concerned for the injury they had done them; nor for the injustice and cruelty they had been guilty of; nor did they fear the wrath of God, and a future judgment; but they were put into a panic, when they found the men they had so ill used were Romans; lest they should be called to an account by the Roman senate, and be found guilty, and have their places taken away from them, and their persons punished.